1. Field of the Invention
The present invention relates to a method for forming semiconductor device having a high environmental resistance enabling it to operate under various operating conditions.
2. Related Background Art
Since the invention of a transistor in 1947, ICs and LSIs are derived from transistors and semiconductor techniques have been remarkably developed. In particular, a silicon semiconductor technique has been established, and various circuits are formed into ICs which are used in a variety of applications. However, elements have operational limitations caused by physical properties of silicon. In some applications, semiconductors other than silicon have been used.
For example, in the field of communications having a very high frequency such as a microwave as in satellite communication, the carrier mobility of silicon is an obstacle against the development of elements. A countermeasure against this problem is a method of precisely micropatterning elements, but the micropatterned element in turn poses another problem regarding allowable power characteristics including heat resistance. For this reason, GaAs having a high carrier mobility has been used in place of silicon for the above applications. Similar problems are posed by high-speed digital circuits (e.g., a supercomputer). For this reason digital ICs using GaAs as a semiconductor material have been developed and are often used in practice.
When silicon is used to form a bipolar transistor, a large current amplification factor, i.e., a high current gain cannot be achieved by a simple pnp layer structure. The collector breakdown voltage cannot be increased by this structure, either. For this reason, a specific structure and circuit arrangement have been implemented for obtaining a larger current amplification factor and a higher collector breakdown voltage according to the state-of-the-art techniques.
A material having a large band gap is preferably used in the above applications. A heterojunction bipolar transistor is assumed as one of the devices which can replace conventional silicon bipolar semiconductor devices. Examples of the heterojunction bipolar transistor are disclosed in Japanese Patent Laid-Open No. 62-216364, 62-265762, 62-160760, and 62-159463. These examples are transistors each utilizing a heterojunction formed between silicon and silicon carbide. However, since silicon is used as a material, drastic solutions to the problems on a low breakdown voltage and a low resistance to environment cannot be provided. Since materials such as diamond and silicon carbide have large band gaps among the currently available materials, good performance can be expected in the above applications if these materials are used. Extensive studies have been made to develop a better semiconductor material using these materials in applications which cannot be handled by the silicon semiconductor, as described in Japanese Patent Laid-Open No. 64-55862 filed by the present applicant.